The present invention relates to a new design for the rear fuselage section of an aircraft, specifically for those aircraft that comprise a horizontal tail plane (HTP) with a box-type central element.
One object of this invention is to provide a rear fuselage section for an aircraft wherein the cut-out required for emplacing the HTP is displaced from a structural part of the rear fuselage section to a non-structural part of said rear fuselage section. Thus, the invention provides a rear fuselage section for an aircraft which is optimal in terms of stiffness and is easy to manufacture.
Another object of this invention is to provide a rear fuselage section for an aircraft that eases the assembly of the section. By dissociating structural and non-structural requirements in the rear fuselage section, the invention enables a rear fuselage section that involves the assembly of continuous frames instead of split frames with joining requirements in a structural part of the rear fuselage section.
Another object of this invention is to provide a rear fuselage section for an aircraft that eases the maintenance tasks of the section. Having a rear fuselage section with the cut-out in a non-structural part of the rear fuselage section, and, providing a detachable fairing in said non-structural part, the invention eases the access to the bottom part of the aircraft. Thus, the invention provides a rear fuselage section that simplifies the maintenance and repair tasks, no longer requiring to remove the tail cone to access the box-type central element of the HTP or the THSA, but detaching the fairing provided to inwardly enclose the HTP or accessing it through access doors.
As it is known, the rear end of the fuselage of an aircraft supports both the vertical and horizontal tail planes (VTP and HTP). Both planes act during standard aircraft operations to trim the aircraft in flight in order to provide stability and control. As is shown in FIG. 1, the rear end of the fuselage comprises two different sections, the tail cone 1 and a rear fuselage section 2. Commonly, the VTP 17 and the HTP 23 are fitted to said rear fuselage section 2.
The majority of conventional HTP's comprise two lateral bodies joined to a central element. The lateral bodies project outwardly from each lateral side of the aircraft fuselage, and the central element, which is joined to both lateral bodies crosses the internal aircraft structure by means of openings in the fuselage skin. There are two kinds of HTP depending on the type of central element they comprise. The central element can be a piece composed of a number of shafts/rods (as shown in FIG. 2a), or of a box-type central element (as shown in FIGS. 2b and 2c), specifically consisting on a center-joint.
For illustrative purposes, FIGS. 2a, 2b and 2c are provided to show a cross section of the aircraft, along the xy plane as detailed in FIG. 1. FIG. 2a shows an HTP which comprises two lateral boxes 5 joined proximately at their edges by a front shaft 3 and a rear shaft 4, wherein both shafts 3, 4 are joined together by an assembly of shafts along the aircraft symmetry plane 25. FIGS. 2b and 2c show two possible embodiments for HTPs with a box-type central element configured to join the two lateral torsion boxes. In both cases, the box-type central element transversely crosses the rear section of the fuselage of the aircraft, requiring a box-shaped opening in the fuselage. FIG. 2b shows a box-type central element 19 whose configuration comprises a central torsion box. FIG. 2c shows a box-type central element configured to join the two lateral torsion boxes 20 along the aircraft symmetry plane 25, wherein said configuration comprises a rib 18. In this last embodiment, the box-type central element is formed by the portion of the two lateral torsion boxes 20 which lies within the fuselage of the aircraft.
The need for the HTP to trim while, at the same time, remaining attached or mounted to the rear fuselage section, means piercing the fuselage skin. FIGS. 3a and 3b show the footprint on the fuselage skin of the openings in the fuselage due to the emplacement of the two types of HTPs.
In the case of an HTP with a central element formed by shafts, the fuselage skin has to be pierced, and two slots have to be provided in the fuselage skin to allow the HTP to trim. These slots enable the joint between shafts and lateral boxes and the trimming of the whole structure. The rear shaft defines the pivoting axis for the HTP trimming, whereas the front shaft is driven by an actuator to perform the trimming movement. Neither the front nor the rear shafts of this HTP cross any frame of the rear fuselage section, so there are no interrupted frames in this kind of HTP assembly. FIG. 3a shows a lateral view of the fuselage skin 27 perforated by the slots 30.
In the case of an HTP with a box-type central element, the rear fuselage section has to be crossed by said central element and, at least, one frame has to be split for emplacing the HTP in its conventional mid-frame position. Said emplacement also entails a full removal of a section panel of the fuselage skin, hereinafter referred to as cut-out, which is needed, not only to attach the HTP to the fuselage structure, but also to allow it to trim. FIG. 3b shows a lateral view of the fuselage skin 27 with a cut-out 7 for emplacing the HTP. The cut-out 7 must be bigger than the box-type central element 19 for allowing the HTP to trim. Since the integration of the HTP with the fuselage skin must comply with severe aerodynamic constraints, the spaces between the walls of the cut-out 7 and the HTP torsion box have to be covered by a dedicated element 32 for ensuring a correct aerodynamic behavior of the area.
Commonly, this dedicated element may consist of an apron or a shield. An apron is made of a deformable material, suitable for adapting itself to the HTP movement, as it is attached both to HTP and cut-out walls and always covers the gaps between said parts. Thus, an apron is more suitable for covering small openings in the fuselage skin where the surface curvature of the rear fuselage is small. Shields, on the other hand, are made of rigid material and move together with the HTP. Shields are more suitable for covering bigger openings, such as those used in wide-body aircrafts. These big openings also usually require modifying the aerodynamic surface of the fuselage skin surrounding the cut-out to reduce the gaps between the leading and trailing edge structures of the HTP and the walls of the cut-out and to provide enough space for said structures to be able to move during the trimming of the HTP. This modification usually implies moving away from a conventional conical section which is easy to remold towards a much more complicated section which poses difficulties in said stage.
Focusing on HTPs with a box-type central element, FIG. 4 shows in greater detail a perspective view of a rear fuselage section, wherein the tail cone is removed and the tail cone attachment fittings 11 appear in the foreground. As has been mentioned, the cut-out 7 for the HTP forces to construct, at least, one of the frames 6 of the aircraft, not as a unitary body, but as separated pieces, joined together by ancillary elements that act as horizontal stabilizer attachment fittings, commonly referred to as swingles 8. Swingles 8 provide the attachment points that create the pivot axis 29 around which the HTP rotates to trim the aircraft.
As the cut-out 7 entails a stiffness reduction in the fuselage skin, the section is reinforced by means of reinforcement 12. Conventionally, said means of reinforcement 12 are stiffening elements disposed in the trace of stringers to frame the cut-out 7 and, the means of reinforcement 12 are joined to the stringers 31 of the fuselage section once the cut-out 7 ends. Said means of reinforcement 12 are highlighted for illustrative purposes, but they are inwardly mounted at the section shown.
Additionally, since the rear fuselage section 2 has to be able to support heavy loads, appropriate elements have to be incorporated to the split frames 6 for achieving a convenient distribution of the loads absorbed by the section. For this purpose, split frames 6 further comprise tie 10 and stiffening beams 13. Frames 6 and swingles 8 are joined together by diagonal support struts 9.
The fastening of the HTP to the rear fuselage section is made by horizontal stabilizer attachment fittings (swingles). Both swingles, each one on the lateral sides of the aircraft, define the pivot axis around which the HTP rotates. The trimming of the HTP is achieved by means of an actuator, called trimmable horizontal stabilizer actuator (THSA). FIGS. 5a and 5b show different views that involve the THSA 15 and its connection with other elements of the rear fuselage section.
FIG. 5a shows a schematic lateral view of the aircraft that details the fastening of the HTP to swingles. Conventionally, at one side, the box-type central element 19 of the HTP is attached to swingles 8 by means of a lug 32, and at its opposite side, to the THSA 15. Through its attachment, the THSA 15 drives the rotating movement of the HTP.
As conventionally, FIG. 5b shows the THSA 15 placed along the aircraft symmetry plane 25, in a forward position with respect to the front spar 34 of the torsion box of the HTP. The fitting of said front spar 34 with the THSA 15, is typically made by means of a jackscrew 33. At each side of the rear spar 35 of the torsion box, close to the fuselage skin 27, the lug 32 attaches the rear spar 35 to the swingle 8.
As it has been mentioned, HTPs with a box-type central element involves several drawbacks that need to be addressed.
There are known solutions that avoid the need for the cut-out for locating the HTP. For example, Spanish patent ES2373812B1 discloses a horizontal stabilizer configuration with a negative sweep angle, where the structural configuration of the VTP and the aircraft fuselage frames are such that it enables the connection of the HTP to the rear end of the fuselage, without requiring the fuselage to have a structural opening. The patent describes a structural connection of the HTP to the aircraft fuselage made between points of the front spar of the HTP and a fuselage frame, such that a structural opening is not required in the skin panels of the fuselage affected by the VTP and HTP load introduction. In this way, that solution avoids a cut-out in a zone heavily affected by the structural loads introduced by VTP and HTP.
However, a new design for the rear fuselage section of an aircraft with a weight reduction and an easier manufacturing and assembly are still desired for an HTP assembly with a box-type central element.